Apparatus for degassing a nuclear reactor coolant system

ABSTRACT

An in-line dissolved gas removal membrane-based apparatus for removing dissolved hydrogen and fission gases from the letdown stream from a reactor coolant system.

BACKGROUND

1. Field

The present invention relates generally to a process for removingdissolved gasses from reactor coolant in a nuclear power plant and moreparticularly to apparatus for removing dissolved hydrogen and fissiongases from the reactor coolant by passing the coolant over a membraneand extracting the gasses by applying a vacuum.

2. Related Art

During pressurized water reactor plant shutdowns, it is a commonpractice to drain the reactor coolant system to a level below thereactor vessel flange to the mid-plane of the reactor vessel coolantoutlet nozzles. That mid-plane coincides with the mid-plane of theconnecting “hot leg” piping leading to the steam generators. Thisdrain-down permits inspection, testing and maintenance, during shutdown,of pumps, steam generators, support structures and other primary systemcomponents.

During reactor operation, some fission gases, e.g., xenon and krypton,created by the fission reactions occurring in the nuclear fuel, mayenter the reactor coolant system and become dissolved in the reactorcoolant. Subsequent to shut down, but before refueling and maintenanceoperations commence, the concentration of radioactive gases and hydrogenmust be reduced to avoid excessive radiation exposure to plantmaintenance inspection personnel and reduce the likelihood of anexplosion due to a potential spark setting off a flammable mixture ofair and hydrogen in the containment atmosphere.

Reactor coolant has previously been degassed using a volume control tankconnected to the reactor coolant system. Generally, the reactor coolantsystem primarily includes such nuclear steam supply system components asthe reactor vessel, the steam generators, the reactor coolant pumps andthe connecting piping. The volume control tank is part of the systemknown as the chemical and volume control system which operates in thedegassing mode by flashing the dissolved hydrogen and radioactive gasesout of the reactor coolant and into the vapor space of the volumecontrol tank. An example of such a system could be found in U.S. Pat.No. 4,647,425.

Typically, a relatively small flow of reactor coolant referred to as theletdown flow is diverted from the reactor coolant system and through thechemical and volume control system. This stream is first cooled thenpurified in a mixed bed demineralizer, filtered to remove dissolvedionic or suspended particulate material and passed to the volume controltank.

U.S. Pat. No. 4,647,425 proposes an improvement to this chemical andvolume control system procedure and reduces the time required toeffectively degas the reactor coolant. The method proposed by the patentprovides for vacuum degassing a reactor coolant system. The methodcomprises draining down the reactor coolant system to approximately themid-point of the hot leg and maintaining the reactor coolant system inan unvented condition during the drain-down operation. Any flashedreactor coolant in the primary side of the steam generator is thenrefluxed. As used in the above mentioned patent, flashed reactor coolantmeans liquid coolant which flashes into the steam phase as a result oflower ambient pressure. Refluxed means condensed and cooled. The bulk ofthe reactor coolant as well as the refluxed reactor coolant, arecirculated through a residual heat removal system to cool the reactorcoolant. A vacuum is drawn on the reactor coolant system to evacuate anygas stripped from the reactor coolant. Preferably, the step of drainingthe coolant system establishes a partial vacuum in the unvented reactorvessel and reactor coolant system during drain-down. The partial vacuumis sufficient to cause the reactor coolant to boil at the prevailingtemperatures in the reactor coolant system whereby the degassing occursduring the drain-down step.

FIG. 1 shows one prior art embodiment of a vacuum degassing system 10that is currently in use. The letdown flow enters the system at theinlet 12 and is directed to an inlet 14 of a degasifier column vessel 16where it enters the interior of the vessel through a spray head 18. Avacuum is drawn on the vessel through conduit 20 by the degasifiervacuum pumps 36. Excess reactor coolant which is not evaporated is drawnfrom the vessel by discharge pumps 22, with pulse dampeners 24 employedto smooth out the pulses generated by the diaphragm discharge pumps 22.The coolant that is drawn through the discharge pumps 22 is exhausted toa holding tank 26 for return to the system or disposal. The water vaporand non-condensable gases that are separated from the coolant in thedegasifier column 16 are routed through a demister 28 to remove anyentrained coolant and conveyed to a vapor condenser 30 in which it isplaced in heat exchange relationship with chilled water that enters andexits the vapor condenser through inlets and outlets 32 and 34. Theradioactive gases and hydrogen are then drawn by vacuum pumps 36 to adegasifier separator 38. The separated coolant is then drawn off by thedegasifier separator pumps 40 and discharged to the holding tank 26. Theradioactive gas and hydrogen are vented from the degasifier separator 38vapor space to the reactor plant radioactive waste gas system 42. Thenitrogen purge line 44 is provided to purge any residual hydrogen andradioactive gases prior to maintenance.

This traditional approach requires significant energy to operate largevacuum pumps, multiple components, e.g., degasifier columns, transferpumps, separator vessels, interconnecting piping, valves, andinstrumentation, and requires significant building space and supportsystems, e.g., cooling/chilled water. Thus, while these systems have along track record, further improvement is desired that will simplify thedesign, reduce the energy required to operate the system, the amount ofbuilding space that is required to house the system and reduce thecapital and maintenance costs of the system.

SUMMARY

These and other objects are achieved by a nuclear reactor power plantsub-system for removing radioactive gases and hydrogen gas from areactor coolant. The sub-system includes a contactor housing a membranethat divides an interior of the contactor housing into an inlet chamberand an outlet chamber, wherein the membrane has pores that pass theradioactive and the hydrogen gases from the inlet chamber to the outletchamber, but prevent the reactor coolant from passing through to theoutlet chamber. A vacuum generator is connected to the outlet chamberfor drawing a vacuum on the outlet chamber. A liquid outlet conduit isconnected to an outlet nozzle on the inlet chamber for conveying adegasified portion of the reactor coolant to a desired location.Similarly, a gas outlet conduit is connected to an outlet nozzle on theoutlet chamber for conveying the radioactive and hydrogen gases to anuclear reactor power plant waste gas system.

In one embodiment, a “sweep” gas system is connected to the outletchamber for supplying a relatively small inert gas purge flow in theoutlet chamber and preferably, the inert gas is nitrogen. The sweep gas,in combination with the application of a vacuum, enhances the efficiencyof the membranes for dissolved gas removal, thus minimizing the requirednumber of contactors. In still another embodiment, the contactor housingcomprises a plurality of contactor housings connected in parallel.Alternately, the contactor housings may be connected in series. In stillanother embodiment, the contactor housing comprises a plurality ofcontactor housings with at least some of the plurality of contactorhousings connected in parallel and some of the parallel connectedcontactor housings are connected in series with at least one other ofthe plurality of contactor housings. In still another embodiment, thecontactors may be operated without a sweep gas, but may requireadditional contactors in series and/or parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic layout of a prior art vacuum degasificationsystem;

FIG. 2 is a schematic layout of one embodiment of the components of thisinvention that replace the portion of the system of FIG. 1 within thedotted lines; and

FIG. 3 is a schematic layout of the system of FIG. 2 with an additionalcontactor housing placed in series with the two parallel arrangements ofcontactor housings to further improve the quality of the output.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention utilizes a known and established technology of gasmembranes to remove dissolved gases from the reactor coolant. While thisis a known and proven technology for some applications, it has not beenpreviously employed to handle mildly acidic and radioactive solutions asexists in interfacing with the primary coolant of a nuclear reactorsystem, as evidenced by the alternative reactor degasing systemsproposed in the past and described in the evaluation of prior art setforth in the Background of U.S. Pat. No. 4,647,425.

In accordance with this invention, one or more alternate “contactors”which respectively house a gas membrane are aligned in series and/orparallel, as required to handle the desired flow and the degree of gasremoval. Liquid containing primarily dissolved hydrogen and theradioactive gases, i.e., xenon and krypton, enters the contactors at arelatively low pressure and exits the membranes degassed to the desiredlevel. A vacuum is applied to the gas side of the membrane to pulldissolved gases from the liquid through tiny pores in the walls of themembrane. In addition, a small inert gas sweep gas, e.g., nitrogen, flowon the vacuum side is used to enhance dissolved gas removal. This gasflow minimizes the number of required contactors. Inlet and outletdissolved hydrogen analyzers monitor the membranes' performance. Such asystem is illustrated in FIGS. 2 and 3. FIG. 2 shows two contactors 46in parallel though it should be appreciated that one, three or four ormore contactors may be employed in parallel as necessary to handle therate of flow that is required. FIG. 3 shows the two contactors inparallel as shown in FIG. 2, with a third contactor in series with theoutput of the two contactors in parallel to further reduce the amount ofgases that may remain within the degassed coolant stream.

Referring back to FIG. 2, the letdown stream enters the system at theinlet 12 and is distributed through inlet conduit 48 to each of theinlets 50 on the contactors 46. A vacuum is applied to the gas side ofthe membrane at the gas outlet 52 by the vacuum pumps 54 and a smallinert gas flow, preferably of nitrogen, is introduced at the gas inlets56 from a nitrogen source 58. By “inert gas” is meant a gas that willnot react with the stripped gasses, i.e., the radioactive gases orhydrogen, to form an undesirable or hazardous gas mixture when vented tothe waste gas system. For example, helium gas may be used, whereasoxygen may not be used. The membrane within the contactor 46 has poressmall enough to prevent the coolant from passing to the gas outlet 52,but large enough to enable the hydrogen and radioactive gases to passthrough the membrane.

Such contactors are available commercially, such as Liqui-Cel, availablefrom Membrana Corporation, Charlotte, N.C. The degasified coolant thenexits the contactor 46 at the outlet 60 and is conveyed by the outletconduit 62 to a holding tank 26 where it can be returned to the reactorsystem or disposed of. As many contactors 46 can be arranged in parallelas necessary to handle as much volume of gas laden coolant as is neededto be recycled or disposed of. The extracted hydrogen and radioactivegases and the nitrogen sweep gas are then circulated by the vacuum pumps54 to the plant radioactive gas waste system 42. The nitrogen source 58also provides flow in the gas lines to purge the gas exit side of thesystem, for maintenance. A source of clean demineralized water 44 isprovided for flushing of the liquid side of the contactors and pipingprior to maintenance.

FIG. 3 is identical to FIG. 2 except an additional contactor 46 ispositioned in series with the parallel arrangement of contactors 46shown in FIG. 2 and provides another stage of degasification to enhancethe purity of the coolant that exits the system. Sensors are providedthroughout the system to monitor the efficacy of the process.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any and all equivalents thereof.

What is claimed is:
 1. A nuclear reactor power plant subsystem forremoving radioactive gasses and hydrogen gas from a reactor coolantcomprising: a contactor housing a membrane that divides an interior ofthe contactor into an inlet chamber and an outlet chamber, wherein themembrane has pores that pass the radioactive and hydrogen gasses fromthe inlet chamber to the outlet chamber, but prevent the reactor coolantfrom passing through to the outlet chamber; a vacuum generator connectedto the outlet chamber for drawing a vacuum on the outlet chamber; aliquid outlet conduit connected to an outlet nozzle on the inlet chamberfor conveying a degasified portion of the reactor coolant to a desiredlocation; and a gas outlet conduit connected to an outlet nozzle on theoutlet chamber for conveying the radioactive and hydrogen gasses to anuclear reactor power plant waste gas system.
 2. The nuclear reactorpower plant subsystem of claim 1 including an inert sweep gas supplyconnected to the outlet chamber for supplying a relatively small inertgas sweep flow in the outlet chamber.
 3. The nuclear reactor power plantsubsystem of claim 2 wherein the inert gas is nitrogen.
 4. The nuclearreactor power plant subsystem of claim 2 wherein the inert gas ishelium.
 5. The nuclear reactor power plant subsystem of claim 1 whereinthe contactor housing comprises a plurality of contactor housingsconnected in parallel.
 6. The nuclear reactor power plant subsystem ofclaim 1 wherein the contactor housing comprises a plurality of contactorhousings connected in series.
 7. The nuclear reactor power plantsubsystem of claim 1 wherein the contactor housing comprises a pluralityof contactor housings with at least some of the plurality of contactorhousings connected in parallel and some of the parallel connectedcontactor housings connected in series with at least one other of theplurality of the contactor housings.